2362 Inorganic Chemistry, Vol. 17, No. 9, I978
Searle, Keene, and Lincoln Contribution from the Department of Physical and Inorganic Chemistry, The University of Adelaide, Adelaide 5001, Australia
Homogeneous and Charcoal-Catalyzed Isomerizations of the (Diethylenetriamine)(methyldiethy1enetriamine)cobalt (111) and Bis( diethylenetriamine)cobalt(III) Ions GRAEME H. SEARLE,* F. RICHARD KEENE, and STEPHEN F. LINCOLN Received September 27, 1977
Each of the s-fuc and u-fuc isomers of [C~(dien)(medien)](ClOd)~ isomerizes in homogeneous aqueous solution to the same equilibrium mixture of the three geometric isomers, with no other products detectable. Proportions after 2 days at 90 "C were s-fuc:u-fac:mer = 81:10:9. This reaction is the basis for obtaining on a preparative scale the new u f u c and mer isomers which have been characterized by "C NMR spectra. Over catalytic charcoal disproportionation occurs to a mixture of seven complexes which can be mostly separated on SP-SephadexC-25 cation-exchange resin. Equilibrium molar proportions s-Juc:u-fac:Co(medien);+ s-fuc = 5:-23:19 [which after 4 h at 80 "C were Co(dien);+ ~-fac:u-fac:mer:Co(dien)(medien)~+ The charcoal reactions must involve complete dissociations includes a little nonseparable mer-C0(dien)(medien)~+]:-23:4:26. of ligands, and by comparison the homogeneous reactions must involve intramolecular processes. In the homogeneous isomerizations at least two rearrangment processes must be involved, and probably three rearrangements occur. Similar experimental results have been obtained for the catalyzed and uncatalyzed reactions of the three isomers of Co(dien)?+. All the homogeneous reactions are base-catalyzed and probably involve conjugate-base intermediates (deprotonated at the secondary amine nitrogens of coordinated dien). All the experimental data for the homogeneous reactions are most satisfactorily accommodated by the proposal of transient five-coordinated intermediates, formed by bond rupture, rearranging. The possibility of twisting mechanisms occurring through trigonal-prismatic transition states is also considered. Racemizations of C ~ ( e n ) under ~ ~ + both charcoal-catalyzed and homogeneous conditions are also base catalyzed.
Introduction Intramolecular rearrangements in octahedral complexes,' manifested as isomerization or racemization processes, can occur by two major pathways: nondissociative, involving the twisting of one octahedral face relative to the opposite face about a C3(or a pseudo- or imaginary-C3) axis via an idealized trigonal-prismatic transition state, and bond rupture via idealized trigonal-bipyramidal or square-pyramidal transition states. In rearrangement reactions where the processes have been established to be intramolecular, the choice between the nondissociative and bond-rupture alternatives has seldom been unambiguous, particularly for complexes of bidentate ligands on which almost all of these studies have been Since the initial proposals of twist mechanism^^^ there have been considerable efforts to establish experimentally their existence, and the work has been reviewed.1,2,s Only recently have twist mechanisms for isomerization and racemization processes in some octahedral complexes been definitely demonstrated and several approaches have been used. Conventional kinetic approaches involving exchange and competition rate measurements do not in general provide sufficient data to distinguish the two intramolecular alternatives. However, the complexes cis- and trans-tetracarbonyl( methoxymethylcarbene) (trialky1phosphine)chromium(O), (CO)4Cr(C[OCH3]CH3)(PR3), R = C2H5, C6Hl1, provide an interesting instance where the cis-trans isomerization was assigned as a twist p r o c e ~ s .For ~ these complexes, with exclusively monodentate ligands, all possible dissociative processes were eliminated by the kinetic data leaving an intramolecular process, that is, a twist in this particular instance, as the only plausible mechanism. Dynamic 'H N M R spectroscopy2 has been employed to establish twist mechanisms for the dynamic inversions A F? A of some tris(tropo1onato) complexes of cobalt(II1) and aluminium by Holm and co-workerslO,ll and of some tris(dithiocarbamato) complexes of iron(II1) and ruthenium(II1) by Pignolet and ~ o - w o r k e r s . Lawrance ~ ~ , ~ ~ and StranksI4 have recently demonstrated that volumes of activation, obtained from rate-pressure measurements, may provide a general criterion to distinguish the bond-rupture and twist mechanisms with some stereochemically rigid complexes. By this means 0020-1669/78/1317-2362$01 .OO/O
the racemizations of C r ( ~ h e n ) , ~ C + ,r ( b ~ y ) , ~Cr(~x)(phen)~+, +, and Cr(ox)(bpy),+ l 5 were shown to proceed by twist mechanisms, whereas Cr(ox),,-, Cr(ox),(phen)-, and Cr(ox),(bpy)involved bond ruptures. These methods have limitations. The dynamic 'H N M R method is restricted to stereochemically nonrigid complexes, and diastereotopic substituent groups on the chelate ligands are required as probes to detect the changes in chirality around the metal centers. The activation volume approach may not be easily applicable to complexes of primary and secondary amines where there is the possibility of deprotonation followed by rearrangement of a deprotonated species. Thus the generality of twist mechanisms is still unkown. It was hoped that the use of complexes of multidentate ligands in mechanistic studies of intramolecular rearrangements might offer more opportunity of distinguishing between the bond-rupture and twist alternatives, since the coupled chelate rings restrict the number of different possible interconversions and because such molecules possess more distinguishable points of reference by which to trace the paths of specific rearrangements. We now describe experiments on the isomerizations of the geometric isomers of [Co(dien)(medien)] (C104)3.15 These hexamine-type complexes are useful for such mechanistic studies because the isomerizations are relatively uncomplicated by competing hydrolysis reactions and because they are closely related to other complexes like C ~ ( e n ) for ~ ~which + racemization mechanisms are still unclear. Although the experiments demonstrate that the isomerizations in homogeneous solution proceed by intramolecular mechanisms, the bond-rupture and twist possibilities are not readily distinguished. However, the bond-rupture route is supported by observations on some similar systems.
Results Identification of the Three Co(dien) (rnedien),' Isomers by 13CNMR Spectroscopy. The complexes C~(dien)(medien)~+, Co(dien)?+, and Co(medien)2+ can each exist in three possible geometric isomers as shown in Figure 1. In two of these geometric forms the two secondary or tertiary amine groups are disposed trans to each other, but in the u-fac isomer these groups are in cis positions. 0 1978 American Chemical Society
Inorganic Chemistry, Vol. 17, No. 9, 1978 2363
Homogeneous and Charcoal-Catalyzed Isomerizations
n
B
mer [co
dien2]3*
yciiow
(+mer [Codien rnedienj3*) O,."Q*
1
medien
u-fac [CO dien2I3+ yeiiow
medien
C-NO-C ~~~
dien
C"3
s-fac
I
c-f,
cH3jfib , C-NH-C
incomplete separation
Wen)
5-faC [Co dim rnedien] yellow
70
60
50
'*
40
d
ppm from SiMe4
Figure 2. 13CNMR spectra in DzOof the geometric isomers of [ Co(dien)(medien)] (C104)3.
The compounds s-f~c-[Co(dien)(rnedien)]~+ and s-fuc[ C ~ ( m e d i e n ) ~ ]are ~ +the major products from the reaction of [ C O ( N H ~ ) ~ C with ~ ] C dien ~ ~ and medien, and their geometric configurations were assigned previously from comparisons of their 13CN M R spectra with the spectra of the three isomers of Co( dien) 23+. The u-fuc and mer isomers of C~(dien)(medien)~+ are formed by isomerization of s-fac-[Co(dien)(medien)] (C104)3 in aqueous solution as described below, and their identities follow from their 13C N M R spectra (Figure 2). In each of these two isomers the nine carbon atoms are, in principle, in stereochemically different environments (each isomer has symmetry CJ. However, the two methylene carbons adjacent to the secondary amine nitrogen of dien in mer-[Co(dien)(medien)] 3+ are evidently in closely similar environments since they give a single resonance peak (6 47.0 ppm from SiMe4) just as for m e r - [ C ~ ( d i e n ) ~ ] ~Similarly, +. the two methylene carbons adjacent to the tertiary amine group of meridionully coordinated medien give a single peak (6 62.8). Thus the four methylene carbons in each ligand in a mer complex give a characteristic 2:l:l peak pattern. There are no equivalences for u-fuc isomers, so that the ~-fuc-[Co(dien)(medien)]~+ isomer must be that which gives the nine peaks, all of similar magnitude (Figure 2). Reactions of s-fuc- and u-f~c-Co(dien)(medien)~+in Homogeneous Aqueous Solution. When s-fuc-[Co(dien)(med i e ~ ~ ) ] ( C l Owas ~ ) heated ~ in water a t 90 "C,and the resulting solution was separated chromatographically on a column of SP-Sephadex cation-exchange resin, three bands of the geometric isomers of C~(dien)(medien)~+ separated as the only products (Figure 3A). The rates of formation of the two new minor isomers u-fac and mer seemed to be variable in different runs in water (pH -5-5.5). We therefore carried out two parallel reactions of the s-fac isomer, one in 0.01 M perchloric acid and one in sodium hydroxide solution adjusted to pH 10.5. After 2 days at 90 "C, chromatography of the solutions showed that the s-fuc had remained completely unchanged in the
s-lac
[Co die",]
lemon (weal
Chromatographic separations on Sephadex columns (Na3P04eluent): (A) equilibrium mixture of Co(dien)(rnedien)'+ in aqueous buffer, isomers, from s-fuc-[Co(dien)(medien)] pH -8; (B) products from s-fuc-[C~(dien)(rnedien)](ClO~)~on charcoal in water. Figure 3.
perchloric acid but had substantially isomerized in the sodium hydroxide solution (pH now 7.5), so that it was clear that the isomerization reactions were inhibited by acid and were apparently base catalyzed. The reactions were subsequently carried out in a buffer solution, 0.05 M 2,4,6-trimethylpyridineperchloric acid adjusted to pH 8.0 at 25 OC, although the actual pH at the elevated reaction temperatures would be substantially less than this, probably around 7 . Separate solutions of s-fuc- and u-fuc-[Co(dien)(medien)] (C104)3in the buffer solution were allowed to react at 90 OC. Samples from the two solutions a t this temperature gave apparently similar proportions of the three constituent isomers after 2 days so that each mixture had essentially reached equilibrium by this time. At 3 days, analysis of the proportions gave identical results from the two solutions of starting s-fuc and u-fac. These equilibrium proportions of C~(dien)(medien)~+ were s-fuc:ulfuc:mer = (81 f 2):(10 f 1):(9 i 1) at 90 O C . Chromatography of a solution of u-fac-[Co(dien)(medien)](C104)3in 0.01 M perchloric acid, which had reacted for 2.5 days at 90 OC, showed that some isomerization to s-fuc had occurred (s-fuc:u-fuc 1:lO by visual estimate), but mer was not detectable. The formation of both of the other isomers from u-fac was clearly acid retarded. The isomerization of the more stable s-fuc isomer under the above conditions provides the most convenient means of obtaining the u-fac and mer isomers. Not only are the yields better than from other reactions we have studied (the dis-
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2364 Inorganic Chemistry, Vol. 17, No. 9, 1978
Searle, Keene, and Lincoln
for chlorides); (3) the equilibrium proportions of the three proportionation of s-fuc-[Co(dien)(medien)] (C104), on C~(dien)(medien)~+ isomers as determined separately (though charcoal as described below, and [ C O ( N H , ) ~ C ~ + ] Cdien ~~ + medien) but these other reactions produce the C ~ ( d i e n ) ~ ~ + a t 90 "C). Further, it is improbable that any significant amounts of u-fuc and mer C~(rnedien),~+ isomers also and it is almost impossible to separate merhave escaped detection. It seems clear therefore that the amount of Co'[Co(dien)(medien)I3+ from mer- [Co(dien),] and difficult (medien),,+ formed is less than stoichiometric. This probably to separate s-fac-[C~(dien)(medien)]~+ from u-fuc-[Coreflects the lower stability of the coordination between co(dien)2]3+ by chromatography on Sephadex." balt(II1) and medien than between cobalt(II1) and dien. This Reactions of s-fuc- and u-fuc-[Co(dien) (medien)13+ over may be viewed alternatively as a greater propensity toward Charcoal. In water with catalytic charcoal, s-fuc-[Codecomposition by C~(medien),~+, probably to cobalt(I1) (with (dien)(medien)] (C104)3gave a mixture of seven complexes charcoal, water, or OH-20as possible reductants) and free which were separated chromatographically into six bands on ligand, than by the C ~ ( d i e n ) ~ ~A+ significant . amount of SP-Sephadex cation-exchange resin using N a 3 P 0 4 eluent cobalt(I1) was present (thiocyanate test) in the solution at 48 (Figure 3B). The reaction was studied at 80 "C because some h despite an oxygen atmosphere being maintained above the information on the equilibrium proportions of the three solution. Some demethylation of s-fac-[Co(medien),] (C10J3 C ~ ( d i e n ) ~ ,isomers + is available for that temperature.18 to s-fuc-[ Co(dien) (medien)] 3+ on the charcoal, as observed Equilibrium was established within 4 h. The identities of the in separate experiment^,'^ would also raise the dien:medien six yellow or orange complexes were established from their ratio. relative orders of separation on Sephadex with Na,PO4, The u-fuc isomer of [C~(dien)(medien)](ClO~)~ disprocompared with those of the authentic complexes determined portionated on charcoal to give the same products in similar separately" and which have been characterized by 13CN M R proportions as from the s-fuc isomer above. In this instance spectroscopy.16 the extent of isomerization u-fuc s-fuc-[Co(dien)(meThe molar proportions of these six products were determined dien)13+ was considerable, and again the amount of the mer (atomic absorption) to be as follows: s-fuc-[ C ~ ( d i e n ),+: ~] isomer formed was probably small. u-fuc-[Co(dien),] ,+:mer-[Co(dien),] ,+(+mer- [ Co(dien)(medien)]3+):s-fuc-[ Co(dien)(medien)] 3+: u-fuc-[ Co(dien)Discussion (medien)] 3+:s-fac-[Co(medien),] 3+ = 5 :-23: 19:-23:4:26. Isomerization Reactions of Co(dien) (~nedien)~'in HomoClearly s-fuc-[Co(dien)(medien)I3+ had undergone disprogeneous Aqueous Solution. In aqueous solution without portionation (in the ligand sense) so that the above proportions charcoal, isomerization of both s-fuc- and u-fuc-Co(dien)should represent the relative stabilities of the various species me die^^)^+ occurred slowly to give the same mixture of the corresponding to equimolar amounts of the two ligands. Some three Co(dien)(rnedien),+ isomers, s-fuc:u-fac:mer= 81:10:9. orange mer- [Co(dien) (medien)] was undoubtedly formed, That no other products were observed under these conditions but this complex chromatographs with yellow mer- [Coimplies that the reactions could not have involved ligand (dier~)~],'in a single band" so that the proportions of these dissociation, since this should lead to disproportionation as in two species could not be directly assessed. However from the the charcoal reaction. Certainly any s-fuc-[C~(medien)~]~+ yellow color of this band and the proportion arguments below, would be readily detectable in the chromatographic separations we believe that the proportion of mer- [Co(dien)(medien)I3+ (Figure 3B). Thus the isomerizations must involve entirely was small (
